App Notes ~ Outdoor Power System Design and Cost Considerations
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Authors
Peter Nystrom
President
TSi Power Corp.
A Comparison of AC Voltage
Regulation Technologies
Background
Automatic voltage regulators (AVR) are used in countries with insufficient
infrastructure for the generation, transmission, and distribution of electricity.
They are also used in industrialized countries where grids are weak and inside
facilities with inadequate electrical wiring. Conventional AVR technology has
not kept pace with the requirements of the sophisticated electronics it protects.
The high-speed VRP AVR developed by TSI Power Corporation addresses the
new reality.
The electrical mains were designed to provide power to linear loads such as light
bulbs and heaters. The power they draw from the mains decreases with supply
voltage, mitigating some of the consequences of low supply voltage. However,
modern power converters used in computing, telecommunications, and industrial
equipment are based on the principle of constant power. Because the current draw
increases when the supply voltage decreases, this process compounds problems
within the distribution systems.
Problems with transmission and distribution are primarily found in developing
countries, which tend to have an inadequate electrical infrastructure. This problem
also exists in industrialized countries, but in a different form. It is typically seen
when weak local distribution systems and/or inadequate electrical wiring in some
buildings create mains voltage stability problems.
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White Papers
A Comparison of Ac Voltage Regulation Technologies
Modern electricity systems are based on high-inertia generation, stiff transmission
backbone, and adequately sized distribution systems to permit very quick clearing of
local faults in order to prevent interruptions upstream. Various regulatory bodies and
standards organizations assume that the mains supply voltage is fairly stable since
specific standards are based on conditions within the European Union and United
States.
Constant power loads, whether linear (if power-factor corrected) or non-linear (such
as power converters), are designed to perform within set limits such as 184–264 V, and
with permanent operation allowed at the nominal voltages of 208, 220, 230 and 240 V.
The same is true for inductive loads, such as air conditioners and other AC motors.
Manufacturers of these devices operate in a competitive international market and will not
over-engineer power supplies using magnetic and semi-conductor components with
higher voltage ratings than actually required by the standard input voltage envelope.
OEMs integrate these products into their systems based on the same limitations.
Because of this, there is a need for added mains voltage regulation when a system
operates from an inadequate mains supply.
One economical way to mitigate such problems is using a modern AVR, such as TSi
Power's high-speed VRP AVR.
The reality is that current AVR technology was developed years ago and has not kept
pace with today's needs. The most prevalent technologies currently in use are of the
servo and tap-switching types. It is the end user's responsibility to assess whether the
mains supply is of sufficient quality to provide power to sensitive equipment. The burden
of selecting a proper solution that meets technical requirements also falls on the end
user, who may or may not be qualified to deal with this, which is why some hire a
consultant to provide recommendations.
In order to shed light on the relevant issues, this white paper will discuss the
advantages and drawbacks of these established technologies while describing
the newly developed, high-speed AVR.
AVR System Requirements
Regardless of technology, an AVR for today's sophisticated equipment must operate
without causing disruption or additional problems to the user's connected load. Such
problems may occur due to the inherent design of an AVR. The following performance
characteristics are essential:
• Voltage correction must begin in 20 ms as power converters typically have a
hold-up time of 20 ms
• Output regulation as a percentage of nominal supply voltage should be precise
• Low impedance to minimize load induced voltage swings
• No breaking of power path during switching
• High efficiency
• Fail safe
• Automatic bypass in case of failure
• Reliability
White Papers
A Comparison of Ac Voltage Regulation Technologies
How the Technologies Compare
Servo
Tap
VRp
Dual
0
50
100
150
200
Figure 1: Response time in ms.
The Electromechanical Servo AVR
An electromechanical regulator uses a servo motor to turn the crank of an internal
variable transformer to change the input voltage to the primary of a series connected
buck-boost transformer. The servo receives its control voltage from the output of a
feedback and control system that monitors the output voltage of the secondary winding
of the buck-boost. The supply voltage is connected to the other end of the buck-boost
and load current returns via the system neutral.
series winding
input
voltage
sensing
output
correction
motor
Figure 2: Electromechanical servo AVR.
The Electronic Tap-switching AVR
An electronic regulator utilizes either an isolation or auto transformer with regulation taps
on secondary winding. The typical number of taps range from three to seven, which
translates into the same number of regulation steps - the more steps, the more precise
the regulated output voltage. The taps are switched either using mechanical relays or
SCRs (for higher-current systems). The switches are controlled by a voltage output from
a feedback and control system comparator, which compares input voltage against a
reference value. The error output causes the switches to change to the appropriate tap.
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White Papers
A Comparison of Ac Voltage Regulation Technologies
input
output
Figure 3: Electronic tap-switching AVR.
Dual Conversion AVR
AC input is connected to a full-bridge rectifier that feeds a high-frequency boost
converter circuit. The secondary of the boost converter forms the DC link that feeds
the input of the DC–AC power inverter. The inverter output voltage and frequency can
be the same as input or be set to a different voltage and frequency.
ac in
dc link
rectifier
ac out
inverter
capacitor
Figure 4: Dual conversion AVR.
High-speed VRP PWM AVR
Its basic topology is a buck-boost transformer with a primary to secondary ratio of 5:1 for
a voltage correction of +/-20% (Figure 5). The input phase is connected to one end of
the secondary, while the critical load is connected to the other end of the secondary with
the current returning to the source via the neutral wire. The control voltage is imposed on
the primary winding by connecting one end of the primary to the incoming phase, while
the other end of the primary is connected to the neutral. In this manner, 20% of the input
voltage will be added to the secondary winding. If the connections are reversed, the
voltage will drop 20% as the magnetic flux imposed by the primary winding is bucking
the secondary winding.
White Papers
A Comparison of Ac Voltage Regulation Technologies
AC input
172 – 276 VAC
hot
two pole
circuit breaker
L1
buck- boost
transformer
surge
protection
and
noise
filter
ground
AC
DC
automatic
bypass
microprocessor
control
208, 220, 230 or
240 VAC +/ - 3 %
L1
VRP
output
noise
filter
L2
neutral
L2
ground
Figure 5: High-speed VRP PWM (pulse-width-modulation) AVR.
The basic idea of VRP is to accomplish this task electronically without the step changes
in voltage that occur when the system regulates. This is accomplished through a
feedback and control system implemented by using a microcontroller. The system
uses internal gate bipolar transistor (IGBT) power switches to form two power stacks.
Rectified voltage is supplied to the inverter. Then, the microprocessor measures the
system output voltage fed back from the system and compares this voltage against a
reference. Corrections are made by varying the duty cycle of PWM pulses. The more
voltage that needs to be added, the longer the duty cycle. The system responds in
20 ms to any changes in mains voltage.
This VRP system has a number of advantages:
• One magnetic component
• Compact size and low weight
• One PCB assembly
• Few interconnections
• Rapid response time
• High efficiency
• Low impedance
• Power to load not interrupted during regulation, therefore no di/dt
• Automatic bypass allows critical load to receive power if electronics fail
Plate 1: 7500 VA single-phase system with buck-boost transformer and heatsink-pcb assembly.
Note: ruler scale in inches.
The VRP system
responds in
20 ms to any
changes in
mains voltage.
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White Papers
A Comparison of Ac Voltage Regulation Technologies
AVR Technology Comparison Table
Technology |
Advantages
|
Drawbacks
Electromechanical Servo AVR
Precise regulation
Low impedance
Excellent stability
High efficiency
Correction takes seconds
Voltage overshoots following
mains cycling
Requires periodic replacement
of brushes
Manual bypass only
Complex design
Electronic Tap-switching AVR
Low cost
Slow correction
High efficiency
Average reliability
Low impedance
Coarse regulation
Interrupts load current when switching
Complex internal wiring
Manual bypass only
Dual Conversion AVR
Precise regulation
Very high cost
Fast regulation
Very low efficiency
No switching of power path
Must be oversized for high-inrush loads
Energy storage
Complexity
Corrects frequency variations
Reliability
High-speed PWM VRP
Precise regulation
No energy storage
Fast regulation
Does not correct frequency variation
High efficiency
Low impedance
No switching of power path
Simple design
Automatic bypass
Reasonable cost
Magnetics Selection
A major advantage of VRP topology is the fact that the buck-boost transformer is not
larger than a 2 kVA isolation transformer for a system with output of 10 kVA and with
regulation of +/-20%. Conversely, a similar transformer with a winding ratio of 10:1 would
be able to handle 20 kVA. The result is an economical approach because magnetic
components are kept to a minimum.
Conventional transformers with conventional grain-oriented (CGO) cores can be used at
the lower power levels where losses due to harmonics from the inverter section are not
high enough to impact efficiency. Of course, core materials such as thin-gauge, Hi-B
White Papers
A Comparison of Ac Voltage Regulation Technologies
materials that tolerate higher frequencies will improve efficiency by reducing AC losses
due to eddy current and hysteresis of the material. Losses due to high-frequency
skin-effect can be controlled by appropriate selection of copper conductor thickness;
such as copper foil and litz wire.
Systems Efficiency
The typical VRP has an efficiency of 96%, but this can be improved by the selection of
better magnetics and slower response time.
Conclusion
The VRP offers a significant improvement on the slow, electro-mechanical AVR, the
electronic tap-switching AVR, and the dual-conversion AVR. It offers many of the
attractive features of these technologies at a reasonable price without trading off
performance parameters. Since the VRP doesn't switch the power path, it does not
cause di/dt problems; Plus it is highly efficient and attractively priced.
About the Author
Peter Nystrom is the president of TSi Power Corporation and has been in the power
conversion industry for over 30 years. He can be reached at: peter@tsipower.com
Based in Wisconsin, USA, TSI Power Corporation develops and manufactures electronic
AVRs, DC–AC power inverters and automatic transfer switches as well as customized
equipment to suit specific needs. Customers include OEMs, telecoms, and
governments.
TSi Power Corporation
1103 W Pierce Avenue
Antigo, WI 54409 USA
Tel: +1-715-623-0636
Fax: +1 715 623 2426
Email: sales@tsipower.com
Toll free: 1.800.874.3160
Web: www.tsipower.com
Copyright © 2012 TSi Power Corp.
WP-A-Comparison-of-AC-Voltage-Regulation-Technologies-2012-06-20.pdf
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